1. Technical Field
The invention relates to a power source, especially for use with a data bus in public transportation, wherein the power source has a first transistor and wherein, in normal operation of the power source, the current emitted by the first transistor is determined by a first resistor on the emitter of the first transistor.
2. Description of Related Art
In some areas of technology, power sources or current sinks with low precision requirements are needed. One example of this is the supply of a data bus, e.g. the response bus of the IBIS and/or VDS vehicle bus, which is used in public transportation. The IBIS vehicle bus is used to control ticket validators, interior displays, etc. in buses or streetcars from a central control unit. The control unit assumes the function of a master; the individual users connected to the bus are slaves. By way of the call bus, the master sends a message to the individual slaves and the slaves report their status back on the response bus. The schematic structure of the bus is shown in FIG. 1. The master has a power source that outputs approx. 100 mA to the response bus. A slave that wants to transmit a message on the response bus, connects the line to ground according to the message to be sent using a transistor (a MOSFET in the figure) and thereby creates a bit pattern on the response bus. The voltage swing of the bit pattern typically lies at 28 V. In or at the master, the bit pattern is evaluated and the transmitted message is extracted.
In this or similar applications, usually simply structured power sources are used that fulfill only low requirements for precision of the output current. Simple circuits comprise one or more bipolar transistors for driving the output current and few circuit elements. During the design of the circuit, among other things, attention must be paid to the maximum power loss in the transistor(s). To prevent overheating, frequently a cooling surface or a heat sink is used for a power transistor. However, because of this the power source becomes much more voluminous and—with the use of heat sinks—the manufacturing becomes more expensive and complicated. To avoid the use of heat sinks, sometimes the current supplied by the power source is distributed to several power transistors and a cooling surface is implemented on the circuit board. In this case, frequently SMD (surface mount device) power transistors are used. Still, a comparatively large cooling surface is necessary, which involves a not inconsiderable space requirement on the circuit board and thus costs. In addition, it is possible that a developer may take the circuit section over to a new circuit board and not provide adequately large cooling surfaces. This causes the risk of a component overload.
Therefore, the present invention is based on the object of designing and further developing a power source of the type named at the beginning that can achieve safe operation of the power source simultaneously with the smallest possible space requirement. In this case, the power source can especially be used with a data bus in public transportation.